1. Field of the Invention
The present invention relates to a semiconductor acceleration sensor that is used in airbag systems, anti-lock braking systems (ABS), and navigation systems of automobiles, other consumer appliances, and the like. More particularly the present invention relates to an acceleration sensor having a self-diagnosis function.
2. Description of the Related Art
In recent years, an airbag system has been installed in an automobile as a standard to improve safety. One of the important devices in the airbag system is an acceleration sensor which detects a shock (acceleration) The acceleration sensor is required to have high reliability, and thus it usually has a self-diagnosis function.
FIG. 4 shows a configuration of a conventional semiconductor acceleration sensor. As shown in the figure, the acceleration sensor usually comprises a sensor chip 110 for detecting acceleration and an ASIC (application-specific integrated circuit) 120 for signal processing such as converting, filtering and amplifying a signal for the sensor chip 110. In the acceleration sensor, a movable electrode (mass body) 113 placed between fixed electrodes 111a and 111b is displaced upon receiving the acceleration, the displacement of the movable electrode 113 causes changes in capacitance between the fixed electrodes 111a, 111b and the movable electrode 113. The acceleration sensor detects the acceleration by converting the changes in capacitance into an electric signal to detect the electric signal.
The capacitance changes detected by the sensor chip 110 are converted into a voltage signal by a capacitance-to-voltage conversion circuit 121 in ASIC 120. The voltage signal is fed to a gain adjustment circuit 125 through a low-pass filter (LPF) 123. The gain of the signal voltage is adjusted in the gain adjustment circuit 125, and a sensor output is obtained after the voltage signal is amplified in an amplifier circuit 127. To the gain adjustment circuit 125, the gain is adjusted by using data 133 for gain adjustment. Also, an offset adjustment circuit 129 adjusts the offset value of the amplifier circuit 127 by using data 135 for offset adjustment.
An acceleration sensor usually has a self-diagnosis function. An electrode 115 is provided opposite to the movable electrode 113 in the acceleration sensor for the self-diagnosis. During self-diagnosis operation, a predetermined voltage is applied to the electrode 115 from the signal processing ASIC 120 such that the movable electrode 113 is then displaced by the attractive static force as if an acceleration is generated. The acceleration sensor detects an abnormality by measuring the changes in the capacitance caused by the displacement.
In order to detect an abnormality with high precision during self-diagnosis operation, the deviation in the sensor output during self-diagnosis operation should be more than a predetermined level. For that purpose, increasing the length of the electrode 115 for self-diagnosis in the sensor chip, or decreasing the distance between the electrodes 113 and 115 may be considered. However, modification of the sensor chip configuration is difficult owing to the limitation of the package size, so that such a method is limited in some degree.
Therefore, a method may also be considered to increase the voltage applied to the self-diagnosis electrode 115 by a voltage-booster circuit 131 provided in ASIC 120. By the booster circuit 131, the source voltage is raised to a predetermined voltage so that a sufficient level of the sensor output for self-diagnosis can be obtained, and then applied to the electrode 115. However, when a voltage beyond the source voltage of the ASIC 120 is required for applying to the electrode 115 for self-diagnosis, the temperature characteristic in the output voltage of the booster circuit 131 become problematic, and thus this method is not sufficient.
The object of the present invention is to solve the above problem and to provide an acceleration sensor which can improve precision of the self-diagnosis.
The present invention provides an acceleration sensor which is provided on a semiconductor sensor chip and detects acceleration based on changes in capacitance between a movable electrode and fixed electrodes. The acceleration sensor includes a self-diagnosis electrode which is composed by a semiconductor chip, and an amplifier for amplifying a detection signal obtained by converting changes in capacitance between the self-diagnosis electrode and the movable electrodes into a electric signal, the signal amplified by the amplifier being output as a self-diagnosis signal. The amplification factor of the amplifier during self-diagnosis operation is made greater than the factor during non-self-diagnosis
The acceleration sensor may further include a section for modifying offset drift in the amplified signal obtained by the amplifier during self-diagnosis operation.
In the acceleration sensor, the amplifier may include an operational amplifier, two resistors which are connected to the operational amplifier to provide a feedback loop, and a switch, and one of the two resistors is connected to the operational amplifier through the switch so that the one of the two resistors can be disconnected.
According to the acceleration sensor of the present invention, a self-diagnosis signal of a predetermined level can be obtained without a voltage booster circuit in the integrated circuit that performs predetermined signal processing by increasing the amplification factor of the amplifier circuit during self-diagnosis operation.
Further, by installing a means for modifying offset drift during self-diagnosis operation in the acceleration sensor, more precise results of self-diagnosis are obtained.
Further, by switching the feedback connection of the two resistors to the operational amplifier, the amplification factors are easily switched between a self-diagnosis operation and a non-self-diagnosis operation.